1. Field of the Invention
The present invention relates generally to a semiconductor memory device, such as a DRAM, and a signal line shifting method.
More specifically, the invention relates to a semiconductor memory device and signal line shifting method using a column redundant system for replacing a data line, to which data of a defective column is outputted, with a remaining normal data line and a spare data line by shifting the data line.
2. Related Background Art
Conventionally, in order to relieve defective bits to improve the yield of DRAMs or the like, a redundant circuit system for arranging a redundant cell array for a specific memory cell array is adopted. In a standard redundant circuit system, a fuse circuit for storing defective addresses is mounted in a chip. Then, when a defective address is inputted, the coincidence of the inputted defective address with a defective address stored in the fuse circuit is detected to output a substitution signal. By this signal, a substitution control for selecting a redundant column or a redundant row in place of a defective column or a defective row is carried out.
However, in this conventional redundant circuit system, it is necessary to establish a one-to-one correspondence between defective columns and redundant columns, and thus there is a disadvantage in that it is necessary to provide many redundant circuits in order to enhance the defective relieving efficiency.
On the other hand, as a redundant circuit system for efficiently relieving defective columns by a smaller number of redundant circuits, a system utilizing a data line shift has been proposed (e.g., Japanese Patent Laid-Open No. 3-176899 and No. 5-101648). In this system, when a defective column address is inputted, a data line, to which data of the defective column should be outputted, is avoided, and the next normal data line is used. Subsequently, a data line shift control for sequentially shifting data lines one by one to connect only normal data lines, which include one spare data line arranged on the end portion of the arrangement of data lines, to data input/output lines is carried out.
However, in the defective column relieving system based on the data line shift, if the starting point of the data line shift corresponding to the defective column address is stored to carry out the shift control, when the number of data input/output lines (I/O lines) is large, the number of selecting signal lines for the shift control is large, and as a result the construction of a multiplexer part for the data line shift is complicated.